An object of the present invention is a process for the correction of the distortion of radiological images acquired with a luminance intensifier tube. It can be applied more particularly to the medical field. It can be implemented either in direct radioscopy or in radiology with digitized processing of the signal representing the image. It relates more particularly to future-generation tomodensitometers in which the detection element will be a luminance intensifier device such as this. Its object is to resolve the problems of morphometry raised by the use of such tubes.
An intensifier tube of radiological images is designed to receive a low-power X-radiation and to convert this X-radiation into a more powerful light radiation that can be more easily detected by a display means, especially by a camera. The reason for the weakness of the X-radiation received must be sought in the need to provide protection, especially in medicine, for patients subjected to examinations with radiation of this kind. This is so especially when such examinations are lengthy, as is the case with tomodensitometry processing operations or processing with digitization of image information elements.
An image intensifier tube essentially has a conversion panel to convert a received X-radiation into a light radiation that is capable of striking a photocathode placed in a position where it faces this panel. The conversion of X-radiation into light radiation is obtained in a known way by providing the panel with caesium iodide crystals. Under the effect of the X-ray illumination, photoelectrons are liberated from the photocathode and move towards the screen. This movement towards the screen is subjected to the effects of an electronic optical system. This electronic optical system tends towards an effect where the impacts of the photo-electrons on the screen correspond to the places on the photo-cathode from which they have been emitted.
The screen is itself of a special type: it re-emits a light image representing the electronic image conveyed by the electrons, and this image itself represents the X-ray image. This light image can then be displayed by any display means, especially a standard camera, so as to be displayed on a display device, especially a device of the television monitor type.
A display system such as this has a major drawback: the revealed image is an image that is geometrically distorted in relation to the X-ray image from which it has originated. This distortion occurs essentially between the photo-cathode, excited by the photons emerging from the conversion panel, and the screen that receives the electron radiation emitted by this photo-cathode. Indeed, during their journey, the photo-electrons are subjected to disturbing effects, notably magnetic effects, due to the earth's magnetic field. If all the photo-electrons were to be affected, during this journey, by one and the same type of disturbance, then correcting the effect of these disturbances at any part of the sequence of images to would be enough to avert problems. Unfortunately, these photo-electrons are highly sensitive to disturbances. And the inhomogeneity of the magnetic field in the places through which they pass is then such as to result in a distortion in the electronic image projected on the screen.
To give a more concrete explanation of the effects of a distortion such as this, it may be said that the image of a straight line interposed between an X-ray tube and an image intensifier such as this will be a straight line in the X-ray image that excites the panel, it will be a straight line in the photon image that strikes the photo-cathode, and it will be a straight line in the electron image that leaves this photo-cathode, but it will no longer be a straight line in the electronic image that gets displayed on the screen. Consequently, it can no longer be a straight line in the light image produced by this screen. The display device placed downline then reveals, so to speak, the result of the distortion due to the non-homogeneity of the earth's magnetic field in the space crossed by the electronic image.
Until now, it has been possible to overlook this type of drawback because the images to be produced have been essentially qualitative and because their quantitative content, namely the exactness of the drawing of the contours of the object revealed, has been a matter of little concern. However, at present, with the development of techniques, it is increasingly being sought to use these images quantitatively. For example, prosthetic fixtures may have to be made from the images obtained. In this case, it would be intolerable to have warped images. Besides, in industrial checking, this type of defect obviates any easy use of image intensifiers such as these in metrology.
Among the deformations or distortions of the image, attention may be drawn to the so-called "pincushion" distortion that arises out of the geometry of the spherical dome of the input face of the tube, namely the upper face of the panel. Attention may also be drawn to the so-called "S" deformation arising out of the deflection of the electronic paths by the magnetic fields, especially the earth's magnetic field. The distortion therefore shows a permanent component, related to a given tube, and a variable component related to the very position of the tube in the earth's magnetic field.
Various processes have been envisaged to reduce the effects of this latter distortion. A first approach, through technological developments, has tried to reduce the effects of distortion, namely the effects of the disturbing magnetic fields. To this end, the image intensifier tubes have been provided with magnetic cladding parts (elements to canalize the magnetic field) that encase the tube. However, this casing cannot cover the conversion panel, and accordingly, disturbing magnetic effects are nevertheless exerted in the vicinity of this panel at the position where they are ultimately the most effective owing to the fact that the photo-electrons liberated from the photo-cathode are moved at very low speeds in the vicinity of this panel.
To complete this device, a process has furthermore been devised wherein a coil for the production of a compensating magnetic field is positioned in the vicinity of the upper face of the tube. A French patent application No. 88 04071, filed on 29 Mar., 1988 has even envisaged the servo-linking of the current flowing through this coil to a measurement of the magnetic field to be compensated for. Despite all its promise, this process gives but imperfect results. The precision of the correction of distortion is insufficient in relation to the applications envisaged, for it too cannot be used to eliminate the "pincushion" effect.
Another process of correction of the distortions has been envisaged. It relates to a parametrical approach. According to this approach, the deformations are modelized on the basis of the knowledge of the geometrical and electro-optical characteristics of the system. This success of this process is conditioned by the precision with which the system to be modelized is known. As an analytical approach, it calls for major simplifications of the model in order to be capable of being computed. These simplifications are such that, ultimately, this process can no longer take account of every phenomenon, especially more complex phenomena resulting from the "S" deformation.